An electric power steering apparatus, for example, is for facilitating the manual steering of a steering wheel by adding a torque based on the rotation of an output rotating shaft of an electric motor to a torque based on the rotation of the steering wheel which is manually operated. In such an electric power steering apparatus, the steering shaft on the steering wheel side and the rotating shaft on the output rotating shaft side of the electric motor are coupled by means of a shaft coupling mechanism (a coupling).
In the case where the steering shaft and the rotating shaft are coupled by means of the shaft coupling mechanism, there is a possibility that an impact at the time of the reversing of the output rotating shaft of the electric motor and the vibration of the brushes of the electric motor are transmitted to the steering wheel through the shaft coupling mechanism and the steering shaft, thereby causing an unpleasant steering feel to the driver. To avoid this, it has been proposed to provide a spacer made of such as rubber or soft resin for the shaft coupling mechanism. However, if the spacer is made softer by placing emphasis on the reduction of the impact and vibration, since the soft spacer is likely to undergo creep deformation, backlash can occur in the shaft coupling mechanism owing to the permanent deformation of the spacer due to the repeated load on the spacer. There is a possibility that this can also make the driver's steering feel unpleasant. On the other hand, if the spacer is made hard by placing emphasis on the durability, the steering feel improves contrary to the above since there is no backlash. However, the steering feel becomes uncomfortable due to the impact and vibration transmitted to the steering wheel, as described above.
Accordingly, there has also been proposed a shaft coupling mechanism for an electric steering apparatus which is comprised of a pair of rotation transmitting members, an intermediate interposed member which is interposed between this pair of rotation transmitting members and is elastically deformable, one coupling base body which is coupled to one rotating shaft, and another coupling base body which is coupled to another rotating shaft. This proposed shaft coupling mechanism is one in which, even if it is used over a long-term period, backlash in the rotating direction is difficult to occur between one rotating shaft, e.g., a rotating shaft coupled to the output rotating shaft of the electric motor, and another rotating shaft, e.g., the steering shaft coupled to the steering wheel, which makes it possible to reduce the transmission to the steering wheel of the impact at the time of the reversing of the electric motor and the brush vibration, and which hence excels in durability and prevents the steering feel from becoming uncomfortable (refer to Patent Document 4).